1. Field of the Invention
The present invention relates to a bipolar transistor, and in particular, to a bipolar transistor used for a voltage controlled oscillator (VCO), an oscillation circuit using the bipolar transistor, and a voltage controlled oscillator using the oscillation circuit.
2. Description of the Related Art
Due to the rapid spread of mobile telephones during the past 10 years, high-frequency device techniques and high-frequency circuit techniques have advanced remarkably. As a result, high-frequency devices such as a mobile telephone terminal are being enhanced in performance, miniaturized, and reduced in cost year after year.
A voltage controlled oscillator (VCO) is an example of a key circuit in the above-mentioned communication apparatuses. The VCO generally is composed of a resonance circuit, an oscillation circuit, and a buffer circuit, and provides a stable high-frequency carrier signal required for communication, even at varying temperatures and the like. FIG. 11 shows the configuration thereof.
In FIG. 11, the VCO is composed of a resonance circuit 111 including a variable capacitor whose capacitance value is varied depending upon an applied voltage Vt, an inductor, and the like; an oscillation circuit 112 for oscillating a signal with a frequency in accordance with the resonance frequency of the resonance circuit 111; and a buffer circuit 113 for amplifying the obtained oscillation signal to output a high-frequency signal RFout. FIG. 11 illustrates one possible example of a VCO. Various circuits including transistors in different types have been proposed. Generally, the circuit shown in FIG. 11 is a representative example.
Herein, balance capacitors Cce and Ccb connected to a bipolar transistor T1 are required in the oscillation circuit 112. The balance capacitors Cce and Ccb are used for minute adjustment of an oscillation frequency, adjustment of an output, and adjustment of a fluctuation of a supply voltage and C/N characteristics (carrier-to-noise ratio) to be an important factor of the oscillation circuit.
The above-mentioned circuits are modularized or implemented in an IC chip for miniaturization of mobile telephones. A discrete (single) bipolar transistor mounted in a small package lowers the cost as well as miniaturize the circuit. A conventional bipolar transistor will be described with reference to FIGS. 9 and 10.
FIG. 9 is a plan view of a conventional bipolar transistor seen from the upper surface of a chip. FIG. 10 is a cross-sectional view taken along a line c-c′ in FIG. 9. In FIGS. 9 and 10, reference numeral 1 denotes an N+/N type collector substrate as a semiconductor substrate; 2 denotes a transistor operation region in which a bipolar transistor is formed; 3 denotes an insulating film formed on the surfaces of the collector substrate 1 and the transistor operation region 2; 4 and 6 respectively denote an emitter lead line and a base lead line formed on the insulating film 3 therethrough from the transistor operation region 2; 5 and 7 respectively denote an emitter pad and a base pad connected to the emitter lead line 4 and the base lead line 6 and subjected to wire bonding during assembly; and 8 denotes a collector electrode formed on the reverse surface of the collector substrate 1. Furthermore, in the transistor operation region 2, a P-type diffusion base 21, a P+ base contact 22, and an N+ emitter 23 are formed. Such a chip configuration will be referred to as a substrate collector configuration.
However, conventionally, in order to constitute the oscillation circuit 112 as shown in FIG. 11, the balance capacitors Cce and Ccb that are discrete devices are externally attached to the discrete bipolar transistor T1. Because of the external capacitors, the Q-value (ratio between an inductance or capacitance component and a resistance component) of the oscillation circuit 112 is decreased, which results in a decrease in C/N of the oscillation circuit 112.